A part of the issue with advertised harddrive space and actual harddrive space is down to the fact that harddrives in shops are advertised with storage information in Decimal (Base 10) as opposed to how computers understand storage, which is in Binary (Base 2).
What is the difference?
Well, let's look at binary figures compared to decimal. These become apparent when you indicate the base figure (sub 2 for binary and sub 10 for decimal)
JEDEC, who sets the Global Standards for the Microelectronics Industry uses the binary definitions of Megabyte and Gigabyte which is exactly how computers generally would see things withinin programming etc. JEDEC states that:
A multiplier equal to 1024 (210).
A multiplier equal to 1 048 576 (220 or K2, where K = 1024).
As you can see, each step up (byte to kilobyte, kilobyte to megabyte...) is in multiples of 1024.
Harddrives are sold in decimal storage figures and these are different.
- 1KB (kilobyte) = 100010 bytes
- 1MB (Megabyte) = 100010 kilobytes ... and so on
As you can see, each step up (byte to kilobyte, kilobyte to megabyte...) is in multiples of 100010.
IEEE/ASTM SI 10‑1997 states that the practice of stating that 1 Megabyte = 1024 kilobytes frequently leads to confusion and is to be deprecated. Further confusion results from the popular use of a "megabyte" consisting of 1 024 000 bytes to define the capacity of the familiar "1.44‑MB" floppy disk. An alternative system is found in Amendment 2 to IEC 60027‑2: Letter symbols to be used in electrical technology - Part 2, and the IEC states that figures represented in base 2 are to be referred to as kibibyte or mebibyte or gibibyte etc.
What is the problem?
In computing, within programming and when referring to data storage within semiconductors etc, everything is in Base 2. When you buy a 1TB harddrive you are buying a harddrive which is stated to have be 1012
bytes storage (1,000,000,000,000 bytes). This causes confusion to those who know about binary data and the true figures within computers and would expect to be buying 1,099,511,627,776 bytes and actually getting 91% of that space.
Where you are short changed is in the following:
Let's say all your files to be stored in that harddrive are exactly 1MB (Base 2) each in size. With 1TB spare:
You should be able to fit 1,048,576 files on a 1TB drive. But you can't.
You are actually only able to fit 953,674 files.
Things are also bad when you are storing digital videos. Let's say at 1GB (Base 2) each. With a 1TB (advertised space) harddrive:
You should be able to fit 1,024 files
You are actually only able to fit 931 files (90 files short)
So, as a quick reference to show the amount that the actual values differ compared to the advertised for each common referenced value:
- Megabyte difference = 48,576 bytes (47KB)
- Gigabyte difference = 73,741,824 bytes (72,013KB or 70MB)
- Terabyte difference = 99,511,627,776 bytes (97,179,324KB or 94,901MB or 92GB)
A case in point is that a hard drive in my computer, advertised at 2TB in size, is actually 1863GB in size according to my computer instead of 2048GB. That is 185GB short (92.5GB per advertised Terabyte).